Electron discharge device and method of manufacture



Patented Aug. 9, 1932 UNITED STATES EMIL GIDEON WIDELL, OF BLOOMFIELDNEW JERSEY, ASSIGNOR TO WESTINGHOUSE I LAMP COMPANY, A CORPORATION OFPENNSYLVANIA.

ELECTRON mscnnnen-nnvrcn AND METHOD on ivrnnurnorunn' No Drawing.

This invention relates to the art of manufactu ring electron dischargedevices and more particularly relates to the art of manufacturingelectron discharge devices employing hotcathodes of the oxide coatedtype.

In the manufacture of such electron discharge devices it is customary toform the cathode coating of relatively stable but thermally dissociablecompounds of the alkaline earth metals in the desired proportion, and toeffect the thermal dissociation thereof during the exhaust procedure inthe manufacture of the device.

The most successful of these thermally dis sociable compounds heretoforeemployed has been the carbonate compounds" such as is described incopending application b Duncan MacRae, Serial No. 587,270, filedveptember 11, 1922, entitled Electron discharge devices and method ofmaking, which application is assigned to the same assigneeas the presentinvention.

The customary procedure in the manufacture of electron discharge devicesemploying this type of coated cathode is toeffect entire evacuation ofthe device and degasifieation of the co-operating electrodes and theglass envelope of the device prior to effecting the thermal dissociationof the carbonates in the coating. The carbonate compounds are thenheated to the dissociationtemperature in the relatively high vacuo byincandescing the heating element of the cathode and the gases evolvedfrom" the coating are removed as liberated. This procedure effects readyand complete breakdown of these compounds. At the conclusion of thedecomposition of the carbonates and evacuation of all the evolved gasesthe usual metallic clean-up agent or getter such as magnesium ormisch-metal is vaporized. within the device and the device sealed fromthe pumps. v 7

Under these general manufacturing conditions the use of easilyoxidizable metals as the heater element of the cathode, such astungsten, molybdenum, has not been successful due to the generallysevere oxidizing conditions existant during the manufac'- turingprocedure. For this reason the noble nmetals such as platinum or thenon-oxidiz- Application filed February 15, 1929. .Serial No. 340,342.

ing metals such as nickel and nickelbase alloys have heretofore beenutilized as the v heater element in oxide coated cathodes.

It has been foundthat the electronic emis sivity of the oxide coatedcathode, prepared by the abovegeneral methodsmaybe materially benefitedby'effecting an ionization of the carbon dioxide gases liberated by thedecomposing carbonate compounds in a manner such as, is disclosed andclaimed in U. 8. Patent 1,648,958 (R. E. Myers) issued November 15,1927, which patent is assigned to the same assignee asthepresentapplication. Thebeneficial results obtained through theionization of the carbon dioxide was as: cribed to the formation of aproportion of ionized carbon monoxide (CO) gas Within the device whichreacted in some unknown manner to benefit the thermionic efficiency offilament.

Prior to the above identified Myers patent there was issued on June 1,1926, to J. E. Harris, U. S. Patent 1,586,558 which disclosed the use ofionized carbon monoxide (CO) gas in the activating of hot cathodes ofthe oxide coated'type. The method of forming the cathode coating is notdescribed in this patent but from associated patents by the sameinventor it is believed to be dis similar from the carbonate methodsutilized in the Myers patent.

It is to be noted that in both of these identified prior art processesthe use of or the application of the ionized carbon oxides to effect theactivation of the oxide coated cathode was made at a time subsequent tothe forming by thermal dissociation or otherwise, of the alkaline earthmetal oxides comnate or to avoid.

It is one of the objects of this invention to improve and simplify themanufacture of electron discharge devices employing hot cathodes of theoxide coatedtype and to im- 7 prove the thermionic efficiency of thesaid hot cathode.

Another object of this invention is to provide a means of liberatingwithin the electron discharge "device a desired proportion of a suitablereducing gas such as hydrogen, carbon monoxide, etc.

Still another object ofthis invention is to provide a reducingatmosphere within electron" discharge devices employing hot cathodes ofthe oxide coated type, during the period in the exhaust procedure thatthe oxidecoating of said cathode is being formed.

- Another object of this invention is to provide a cathode coatingcomprised of alkaline earth metal compounds and comprised at least inpart of a thermally dissociable com: ponent which liberates on thermaldissociation a proportion at least of a gas reducing with respect tometallic oxides.

Other objects and advantages will become apparent as the invention ismore fully disa closed.

In accordance with the objects of my invention I have determined thatthe beneficial results heretofore obtained upon the thermionicefficiency of hot cathodes of the oxide coated type by the processes ofMyers and Harris hereinbefore identified, was due primarily to thereducing action of the ionized carbon monoxide gas generated or presentduring the ionization step in the respective processes, upon the cathodecoating or upon the surface oxides of the metal parts associated withsaidcathode either in the cathode electrode or in the co-oper atingelectrodes within the electron discharge device incorporating saidcathode.

There are many theories which have been advanced to explain the effectproduced upon the hot cathode by the ionized carbon monoxide and carbondioxide gases and the true explanation thereof is not at this timeapparent, but whatever the theory that may beinvolved or the trueexplanationof the reactions occurring, I have found that I maysubstantially avoid or eliminate the ionizing feature of the prior artseasoningor activating processes above identified by the provisionwithin the device of a means of liberating a suitable or desiredproportion or amounts of areducing gas such as hydrogen, carbonmonoxide, at the time of or simultaneously with the thermal dissociationof the alkaline earth metal compounds comprising the cathode coating.

.- I have further determined that by providelement or said refractorymetal surface and more particularly I have determined that I may readilyand easily employ such heater elements as tungsten and molybdenum forsuch cathodes and may apply said oxide coating directly to the surfacethereof without the deleterious efiects upon the thermionic efficiencyof the hot cathode that heretofore has been observed.

There are many ways of efiecting the liberation of the desired reducinggas orof jintroducing the said gas into the device during thedecomposition of the cathode coating, such as by incorporating athermally dissociable compoundsuch as the carbonyl compounds, metalhydrides, rare earth metal oxalates, etc., in thermal relationship toone of the co-operating electrodes of the device, or by introducing astream of such reducing gas from an exterior source in a mannerwellknown and practiced inthe incandescent lamp industry in the manufactureof gas filled incandescent lamps, and glowing said cathode thereinl Bothof these broad methods have commercial disadvantages which it. is one ofthe objects of this invention to overcome. i

I have found that for the purpose of my invention, the thermionicefliciency of an oxide coated cathode, is materially increased andaugmented if the evolution of the desired reducing gas takes placesimultaneously with and in intimate association with the decomposingalkaline earth metal compounds of the coating.

I therefore prefer to introduce the'reducing gas by the thermal,dissociation of a chemical compound which is incorporated within orthermally associated with the alkaline earth metal compounds comprisingthe cathode coating. This also proves a ready and'easy manner ofintroducing the desired reducing gas, at the'time of and during theperiod the gas is desired.

Those compounds which I have found useful for the purpose of myinvention are the oxalate compounds of the more strongly basic elementswhich form refractory oxides, which are non-reactive with respect to thealkaline earth metal oxides ultimately formed in the cathode coating andto the oxides of the metallic heater element thereof and non-reducibleby the metallic components of said 'hot cathode. I may also employ ifdesired metallic compounds such as ca-rbonyls, l

h'ydrides, etc., the metal component of which,

such as nickel, is reactive with respect to alkaline earth metal oxidesin vacuo during the operating life of the cathode to liberate thealkaline earth metal of the said oxide compound for electron emissionpurposes.

I prefer however for the purpose of this invention to utilize the abovementioned oxallate compounds and as the easiest and most simple-specificembodiment of my invention will describe the process I employ in themanufacture of electron discharge devices. utilizing a. hot cathode ofthe oxide coated type, in which the cathode coating is comprised atleast in part of the oxalate such. alkaline earth metal oxalates may bereadily prepared or obtained upon the market in a substantially purecondition, are of similar physical and chemical characteris tics to thecarbonate compounds of the alkaline earth metals and may be substitutedin part or in whole for the alkaline, earth metal carbonates heretoforeemployed without requiring alteration-or substantial variation in theusual coating procedures employed in the art.

The amount of alkaline earth metal oxalates added to the usual carbonatecompounds may be Varied quite widely dependent upon the particular metalbase or core member on which they are to be coated and upon the specifictype of cathode being formed, such as directly or indirectly heated, andwill also be influenced somewhat by the amount of coating applied andthe particular type of V in tube within which the cathode is to beutilized, aswell as by other lesser factors.

One ofthe specific and marked advantages that may be obtained by thepractice of my invention is that it provides a means of producing anoxide coated cathode which is comprised of a tungsten filament heaterelement having the oxide coating adjacent the surface thereof, and anoxide coated cathode of the indirectly heated type which employs atungsten filament as the heater element therein, with or without theinter-position of a refractory insulator member between the said heaterelement and the electron emitting oxide coated metal surface.

As a specific embodiment of the practice of my invention I will describethe specific com-- heated oxide coated cathode.

bination of materials I employ in the cathode coating and themanufacturing process I employ in the production of an electrondischarge device employing a hot cathode of the oxide coated typeutilizing a'tungsten filament as the heater element thereof and havingsaid coating applied directly to the surface thereof. I

Heretofore in the art there havebeen' but two methods devised for theuse of tungsten filament as the heater elementof a directly One of thesemethods is the so-called barium azicle process which provides that thetungsten filament be first oxidized and then'this oxidized surface isreduced by vaporized barium metal formed Within the evacuated electrondischarge device by the thermal dissociation and decomposition of bariumazide which has been in corporated therein inthermal relationship to Ithe anode of the device. The volatile products of the decomposition ofthe barium azide are removed b-yevacuation or by suitable getters. Thesecond method utilizes the tungsten filament as a heater element butprovides thereon an interposing substantially stable metallic coatingsuch as nickel, chromium or the non-oxidizing noble metals.

The nickel or noble metal coated filaments have been employed where thedirect application of the coating to the tungsten filament is desiredbut in such composite cathodes the temperatures to which such a cathodemay be incandesced is limited :to the vaporization point of theinterposed nickel or noble metal coating. The chromium coated or othernonoxidizing tungsten filament has been employed in the art as a heaterelement in the indirectly heated type catho'deas it has been found thatsuch coatings are essential to protect the incandesced filament from theoxidizconditions present during the decomposition of the alkaline earthmetal compounds comprising the cathode coating.

When the cathode coating is applied di rectly to the surface of thetungsten I prefer to utilize a mixture of alkaline earth metalcarbonates and oxalates which contain relatively a larger proportion ofoxalates than carbonates, or I may apply to the surface of the tungstena coating comprised entirely of the oxalates if desire The specificcombination employed will depend upon existing manufacturing conditions,the efiiciency of the exhaust, the size of filament, depth of coatingand the particular device within which the cathode is to be employed. Itis relatively immaterial from an electronic or thermionic eliiciencystandpoint as to the specific combination employed except that the ratioof the barium to the strontium and calcium oxides be maintained in sub-7 sta ntially the same proportions as heretofore found effective forbest results, namely barium and strontium oxides in approximately about.0007 inches.

equal proportions with calciumoxide, if present, in amounts up to 10 to20 per cent by weight of the whole.

It is commonly believed that the barium component of the filament,either as free barium metal or as the barium oxide, is the trueactivating or electron emitting constituent of the coating under thepresent operating temperature conditions of this type of oxide ode andsaid devicehaving substantially the same thermionic and electricalcharacteristics as is developed in the type tube known to the trade as a201A radio receiving tube.

In such a device the filamentary heater element must be of the propersize so that 105 millimeters length at 6.0 volts will give the properthermionic operating temperature for the cathode coating. This requiresthat the filament diameter should be approximately Such a small diameteris exceedingly difficult to coat by the drag methods disclosed in theabove identified copending application by Duncan MacRae Serial No.587,270, due to surface tension effects. Moreover with the relativelyhigh percentage of oxalate in the coating composition it is undesirableto expose the same to the heating step of the drag method of coating.

The coating composition is therefore preferably suspended in the organicsolvent such as amylacetate,diethyl carbonate, diethyl oxalate etc.,with or without the addition of proportions of lower boiling alcohols.To this solvent is added a small proportion (approximately 3.0 per cent)of an organic binder material such as intro-cellulose. This type offluid suspending medium is non-reactive with respect to either thetungsten filament core member of the cathode or to the components of thecathode coating, and moreover, may be applied readily to the metalsurface without the necessity of applying heat during the coatingoperation.

The specific admixture I prefer to use with this small diameter filamentis comprised entirely of approximately equal amounts barium andstrontium oxalates. This. admixture is used because with this sizefilament there is a limit to the amount of coating that maybe applied tothe surface so as to obtain the requisite heating effect under thecurrent velope.

rating conditions specifiedfor this type tube. As it is highly desirableto prevent any surface oxidation of the tungsten filamentary coremember, it is advisable to utilize as much of the oxalate compound aspossible. To this admixture, however, I may add proportions of alkalineearth carbonates if desired.

The suspension of the alkaline earth metal compounds are then applied tothe metal surface in the usual manner such as by dipping, spraying orpainting thereon. Prior to coating, the surface of the tungsten filamentshouldv be preferably cleansed and straightened by subjecting the sameto an oxidizing and then a hydrogen fiashing? or annealing operation inthe customary manner. This operation facilitates subsequent handling andmounting operations in the assembly of the electron discharge device. Ihave however, formed cathodes from the black, unannealed drawn filamentand have noted no deleterious effects. due to the presence of thegraphite drawing lubricant thereon, upon the thermionic efficiency ofthe hot cathode subsequently produced. 7

Following the coating of the cathode the same may be mounted in spacedrelation to the co-operating electrodes, sealed into a glass envelopeand the entire assembly subjected to the usual exhaust procedure. At theconclusion of the exhaust, the glass envelope and the co-operatingelectrodes are suitably degasified and a suitable.getter such asmagnesium vaporized within theen- Theftungsten heater element is thenincandesced by the passage of an electric current therethrough and thegases evolved are evacuated as rapidly as possible.

The reaction involved in the thermal dis sociation of the oxalates issubstantially as follows:

Ba(Sr.Ga) C O +Heat= Ba(Sr.Ca) 0+ CO +CO.

Approximately equal amounts of carbon dioxide and carbon monoxide areformed and the total volume of gases is approximately. double in volumeto thatwhich would be obtained by the use of carbonate compounds alone.This increase may be readily taken care of by proper adjustment of therateof exhaust or of the time interval to allow for the increased volumeof gases.

Following the complete evacuation of the device and elimination of theevolved gases from the heated cathode the device may be sealed off andbased in the usual manner. It will be found that the oxide coatedtungsten cathode is substantially thermionically active withouteffecting the so-called ionization steps of the priorart processes andthe thermionic efiiciency thereof will be of an extremely high order andthe life and efficiency equally as good as the best oxide coated cathodeheretofore prepared using nickel, platinum or other non-oxidizing metalbases. It is believed that the reason for the successful application ofthis invention to the forming of an oxide coated tungsten filamentarycathode is substantially due to the presence of the reducing gas carbonmonoxide at the time of and simultaneouslywith the formation of thestrongly basic alkaline earth metal oxides in the coating. It isbelieved that the presence of this reducing gas substantially preventsthe formation by oxidation of the strongly acidic tungsten oxides uponthe surface of the heater element and consequent interaction of thebarium, strontium and calcium oxides with t -e tungsten oxides to formthermionically inactive compounds such as the tungstates,'IZOOlYbClELlZGS, etc., which may not subsequently be thermallydecomposed by heating to higher tempera: tures. t

In this manner there is formed or main tained a substantially oxide freemetal surface upon the heater element of the cathode for the stronglybasic alkaline earth metal oxides to deposit upon. It is apparent thatwhereas in the specific embodiment I have described the use of atungsten heater element, other metal heater elements would be similarlyacted upon.

In the past there has been considerable confusion regarding the effectof, the advantage of or the use of an oxidized surface upon the heaterelement of an oxide coated cathode.-

Whereas in the past it did not appear that such an oxidized surfacedeleteriously effected the thermionic activity of an oxide coatedcathode having a nickel heater element, it did deleteriously efiect thethermionic efiiciency of the same, requiring a greater wattage input tothe heater element in order to obtain therefrom the same milli-amperesper square milli-meter surface area as may be obtained from the sameoxide coated metal surface which is substantially oxide free or brightlymetallic in appearance.

The advantages of the practice of this invention upon a nickel heaterelement is not at once apparent and may only be detected with respect tothe efiect produced upon the thermionic efiiciency of the cathode. lViththe more easily or readily oxidized metal particularly those formingstrongly acidic oxides the efiect is more pronounced, and is amplyillustrated by the specific embodiment herein described.

As an additional application of the inventive feature of the presentinvention mention may be made of the procedure that may be followed inthe manufacture of an oxide coated cathode of the indirectly heated typesuch as is described and shown in copending application entitledElectron discharge device with indirectly heated cathode, Serial In said'copen'ding application the cathode No. 292,116 filed by John Marden andstructure 'is substantially comprised 'of a nickel cylindrical sleevecoated exteriorly with the alkaline earth metal carbonate compoundsandheated by radiation by a tungsten filament centrally disposedwithin saidnickel cylinder. Following the usualexhaust procedure the carbonatecompounds in the cathode coating are decomposed by incandescing thetungsten filament and heating the nickel sleeve by radiation. During thedecomposi tion the heated filament is subjected to erosion by oxidationwhich in some cases materially reduces the'diameter thereof and altersthereby the current carrying characteristics.

By the practice of my invention and the provision within said cathodecoating of a proportion of alkaline earth metal oxalates, the tungstenheater element then may be incandesced in a reducing atmosphereanderosion of the filament reduced to a minimum, and the filamentcharacteristics maintained highly constant.

Another application of the present invention may be noted in themanufacture of elec tron discharge devices of the indirectly heated typeemploying refractory insulators interposed between the heater elementand the nickel sleeve. The Marden and Lederer copending applicationabove identified was di- V rected to a device in-Which this insulator isnot employed. However when such an insulator is employed it is customaryto coat the'heater element with a non-oxidizing metal such as chromiumin a manner such :asdescribed in copending application by JohnHumphreysliamage Serial No. 276,340, filed May 9, 1928, entitled Heaterelement for electron emission device. Said copending application isalso-assigned to the same assignee as the present application.

Through the application of the present invention and the provisionwithin the device of a gas reducing with respect to the tungsten, thedeleterious erosion of the filament during the decomposition of thecathode coating to (eliminate Which erosion the chromium coating hasheretofore been used) is substantially prevented and the filamentcharacteristics of this type heater may be maintained highly constant.

It is apparent that there may be many variations made in the specificembodiment of the present invention without essentially departing fromthe nature of the same as set, forth in the following claims.

What is claimed is: I i i 1. The method of forming a thermionicallyactive alkaline earth metal oxide coating on a tungsten metal surfacewhich comprises coating saidsurface initially With thermallydissociab'le compounds of said alkaline earth metalsvcontaining aproportion of a compound thermally dissociable at least in part to yielda reducing gas during formation of the oxide coating by thermaldecomposition of the initial coating and thereafter effecting thermaldissociation of the coating.

2. The method of manufacturing an-electron discharge device. having anelectron emitting cathode of the oxide coated type a which comprisescoating said cathode atleast in part with alkaline earthmetal oxalates,

- incorporating the coated cathode in the device, removing theatmospheric gases from the device, incandescing the cathode to effectthercoating including an alkaline earth metal mal decomposition of thecathode coating, removing the evolved gases, vaporizing there-. in asuitable metallic sealing 01f the device.

3. A coating mixture for use in forming hot cathodes of the oxide coatedtype comprised of alkaline earth metal carbonates and a pro portion ofan oxalate compound of one of I the strongly basic elements.

4. The method of preventing surface oxidation of a metallic heaterelement of a thermionic cathode-of the oxide coated type during theformation of the coating by decomposition of alkaline earth compounds onsaid heater element composed of a metal in a class comprising tungstenand molybdenum which comprises applying to said heater elementa oxalatecapable ofliberating a gas, reducing With respect to surface oxides oftungsten and molybdenum and thermally dissociating said coating wherebythe reducing gas liberatedprevents the formation of said surface oxides.V

In testimony whereof, I have hereunto subscribed my name this 5th dayof" February,

EMIL cannon WIDELL.

getter and thereafter

